A number of different processes for oxygen delignification have been disclosed. U.S. Pat. No. 4,259,150 presents a system involving a multistage oxygen bleaching in which the pulp is, in each stage, firstly mixed to a lower consistency with O2, water and NaOH, followed by a thickening back to the consistency level which the pulp had up until the stage in question. The aim is to achieve an economical, chlorine-free bleaching with a high yield. At the same time, the kappa number can be lowered, by means of repeated stages, from 70 down to 15, or even to less than 15.
SE-C 467 582 presents an improved system for the oxygen bleaching of pulp of medium consistency. By means of the temperature control having been optimized, an oxygen bleaching takes place in a first delignification zone at low temperature, followed by a second delignification zone which is at a temperature which is 20-40 degrees higher. The aim was to obtain an improved yield and an improved viscosity, while retaining the same dwell time, in connection with industrial implementation.
Besides SE-C 467 582, other variants of oxygen delignification in two stages have also been patented. SE-C 505 147 presents a process in which the pulp is to have a high pulp concentration, in the range of 25-40%, in the first stage and a concentration of 8-16% in the second stage, at the same time as the temperature in the second stage is to be higher than, or the same as, the temperature in the first stage, in line with the temperature difference which is recommended in SE-C 467 582. The advantages of the solution in accordance with SE-C 505 147 are stated to be the possibility of admixing more oxygen in the first high-consistency stage without the risk of channel formation but where, at the same time, unused quantities of oxygen can be bled off after the first stage for further admixture in a second mixer prior to the second stage.
SE-C 505 141 presents yet another process which is an attempt to circumvent SE-C 467 582 since that for which a patent is sought is stated to be the fact that the temperature difference between the stages does not exceed 20°, i.e. the lowest suitable temperature difference patented in SE-C 467 582, but that a temperature difference should nevertheless be present. In addition to this, it is stated that a) the pressure should be higher in the first stage and b) that the dwell time is short in the first stage, i.e. of the order of size of 10-30 minutes, and c) the dwell time in the second stage is longer, i.e. of the order of size of 45-180 minutes.
A lecture entitled “Two-stage MC-oxygen delignification process and operating experience”, which was given by Shinichiro Kondo, from the Technical Div. Technical Dept. OJI PAPER Co. Ltd., at the 1992 Pan-Pacific Pulp & Paper Technology Conference ('99 PAN-PAC PPTC), September 8-10, Sheraton Grande Tokyo Bay Hotel & Towers, presents a successful installation which involves two-stage oxygen delignification and which was constructed in 1986 in a plant in Tomakomai. In this OJI PAPER plant in Tomakomai, the pulp was fed, at a pressure of 10 bar, to a first oxygen mixer (+steam), followed by an aftertreatment in a “preretention tube” (prereactor) involving a dwell time of 10 minutes in which the pulp pressure is reduced to a level of about 8-6 bar due to pipe losses, etc. After that the pulp was fed into a second oxygen mixer, followed by an aftertreatment in a reactor at a pressure of 5-2 bar and with a dwell time of 60 minutes. At this point it was stated that preference would have been given to having a “preretention tube” which would have given a dwell time of about 20 minutes but that it was not possible to achieve this due to lack of space. OJI PAPER stated that, by using this installation, they were successful in achieving an increase in kappa reduction for a lower cost in chemicals and also an improvement in pulp viscosity.
The greater part of the prior art has consequently been aimed at a higher pressure in the first reactor at a level of about 6(8)-10 bar. A pressure in the first reactor of up to 20 bar has even been discussed in some extreme applications. This entails the reactor spaces which are required for the first delignification zone having to be manufactured so as to withstand these high pressure levels, with the attendant requirement for substantial material thickness and/or good material qualities, resulting in an expensive installation.
In pulp suspensions used in industrial manufacturing processes, there are large quantities of readily oxidizable constituents/structures which react even under modest process conditions. It is therefore advantageous to add oxygen in a first stage in quantities which are such that this relatively readily oxidized part of the pulp is allowed to oxidize/react first of all. Severe problems arise if an attempt is made to compensate for this by adding too much oxygen, since there is the imminent problem of channelling (as mentioned in the said SE-C 505 147).
One aim of the invention is to avoid the disadvantages of the prior art and to obtain an oxygen delignification of increased selectivity.
The invention permits an optimal practical application of the theories regarding a first rapid phase and a second slower phase during the oxygen delignification process, where the optimal reaction conditions are different between the phases.
At the high hydroxide ion concentrations and high oxygen partial pressures which are conventionally employed in the first stage, the carbohydrates are attacked more than necessary, thereby impairing the quality of the pulp.
A lower oxygen partial pressure, and preferably a lower temperature as well, in the first stage than in the second stage decreases the rate of reaction for breaking down carbohydrates more than it decreases the rate of reaction for the delignification, thereby leading to an increased total selectivity on the pulp after the two stages.
Another aim is to allow the process installation to be simpler and cheaper, with it being possible for at least one pressure vessel in a first delignification zone to be manufactured using less robust material and/or a lower material quality which is suitable for a lower pressure class.
Yet another aim is to optimize the mixing process in each position such that only that quantity of oxygen is added which is consumed in the following delignification zone. This makes it possible to dispense with bleeding systems for surplus quantities of oxygen at the same time as it is possible to reduce the total consumption of oxygen, which in turn reduces the operating costs for the operator of the fibre line and consequently shortens the pay-off time.
Yet another aim is to increase, in an oxygen delignification system having a given total volume of the first and second stages, a so-called H factor by running the first stage for a short time at low temperature and the second stage for a longer time at a higher temperature. When, for example, carrying out conversions of existing single-vessel oxygen delignification stages, a simple new construction with a small prereactor, and a modest increase in the reaction temperature in the existing reactor, can increase the H factor and at the same improve the selectivity over the oxygen stages.